Advancing climate models: New findings on black carbon’s refractive index and its impact on solar radiation absorption

Our modern society releases a wide range of pollutants into the environment, with combustion being a major contributor, especially in terms of aerosol mass containing black carbon. While black carbon constitutes a relatively small portion of aerosol particles, it poses significant concerns due to its capacity to absorb heat and hinder the reflective properties of surfaces like snow. Therefore, comprehending the interaction between black carbon and sunlight is crucial for climate research.

Recently, researchers have made significant progress in quantifying the refractive index of black carbon, which could have implications for climate models. Black carbon, essentially soot, excels at absorbing solar radiation and trapping heat, thereby contributing to atmospheric warming. Moreover, its presence on lighter surfaces, including snow, diminishes their ability to reflect heat back into space, as darker colors are less effective at reflecting light.

Assistant Professor Nobuhiro Moteki from the University of Tokyo’s Department of Earth and Planetary Science emphasized the fundamental importance of understanding how black carbon interacts with sunlight. He stated, “The refractive index of black carbon plays a crucial role in this context, as it determines how incoming light rays are redirected and dispersed. However, previous measurements of black carbon’s refractive index were not precise enough. To address this, my team and I conducted meticulous experiments to enhance the accuracy of these measurements. As a result, we now estimate that current climate models may be underestimating the absorption of solar radiation by black carbon by a significant 16%.”

Transmission electron microscope images of ambient aerosols collected by an aerosol-impactor sampler installed on the research vessel Shinsei Maru. Red arrows indicate individual black carbon aggregates, most of which were mixed with sulfate (green arrows) and/or organic materials (light blue arrows). Credit: Moteki et al. CC-BY

In summary, black carbon aerosol particles, originating from combustion processes, have received less attention in the media compared to other contributors to climate change, such as carbon dioxide, sulfur dioxide, or methane. Nonetheless, their impact on the climate is substantial. By absorbing solar radiation and impeding the reflection of heat from surfaces, black carbon affects the Earth’s energy balance. The improved understanding of black carbon’s refractive index obtained through recent research highlights the need for accurate representation of this factor in climate models to provide a more comprehensive understanding of climate change dynamics.

Previous attempts to measure the optical properties of black carbon were often hindered by various factors, including the unavailability of pure samples and challenges in studying the interactions of light with particles of different complex shapes. However, Assistant Professor Moteki and his team devised a method to address these issues, significantly improving the accuracy of measurements. They achieved this by capturing black carbon particles in water and isolating them using water-soluble chemicals like sulfates. This isolation process enabled a more precise analysis of the scattering behavior of the particles, providing the necessary data to calculate the refractive index.

Moteki explained, “We measured both the amplitude and phase of light scattered from black carbon samples that were isolated in water. This allowed us to determine the complex refractive index of black carbon, which is not a single number but a value comprising two parts—one of which is an ‘imaginary’ component related to absorption, despite its very real impact. Complex numbers with imaginary components are quite common in the field of optical science and beyond.”

The improved optical measurements of black carbon have implications for climate models, as they suggest that the current models underestimate the contribution of black carbon to atmospheric warming. Moteki and his team hope that their findings will be utilized by other climate researchers and policymakers. Additionally, the method developed by the team to determine the complex refractive index of particles can be extended to study other materials beyond black carbon. This opens up opportunities for identifying unknown particles in the atmosphere, ocean, or ice cores based on their optical properties. Furthermore, the technique can be employed to evaluate the optical characteristics of powdered materials, extending its applications beyond the immediate context of climate change.

Source: University of Tokyo

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